Recent years have seen the rapid development of CCD image sensors and their present widespread use in imaging systems for both amateur and professional applications. Their small size, electrical efficiency, cost effectiveness, etc., have made CCD image sensors the imaging units of choice not only for inexpensive consumer camcorders, but for more critical uses where much higher picture resolution is needed. Depending on their intended uses, CCD image sensors are designed either for sequential (progressive) line-by-line readout of a vertical frame of their image signals, or instead for "interlaced" readout of image signals of first the odd-numbered lines of a vertical frame and then of the even-numbered lines in accordance with a television standard. Where a CCD image sensor is intended for use in much higher resolution imaging than is provided by standard television systems, such as in the electronic printing of color photographs, the CCD image sensor is normally designed for sequential, line-by-line-readout of its image signals. This purely sequential mode of operation is not directly compatible with the "interlaced" mode required for a standard television display.
In order to facilitate the display of video images produced by a CCD sensor on a standard television (TV) display, it is desirable for the CCD sensor to operate in accordance with the same "standard" that the TV display uses. In the United States and a number of other countries the "standard" used for TV is that established by the national television standards committee (NTSC). In Great Britain, Germany, and certain other countries the "standard" is PAL ("phase alternation by line"), while in France and many countries in Eastern Europe the "standard" is SECAM ("sequential chrominance and memory"). While there are differences among the various standards, they all require the display of TV pictures in the form of rapidly scanned horizontal lines of vertical frames. Each vertical frame of a picture represents an instantaneous "snapshot" of the scene being imaged and the frames are displayed in rapid succession as in a motion picture. To further minimize visually apparent flicker in the displayed pictures, each frame thereof is made up of an "odd" and an "even" field superimposed on each other in rapid succession. The horizontal scan lines of an "even" field are precisely interlaced with the horizontal scan lines of an "odd" field, and so on. The NTSC "standard" specifies "525" horizontal scan lines per vertical frame, with "2621/2" lines for each of the "odd" and the "even" fields. This seemingly complicated way of displaying television images is an outgrowth of the development of commercial broadcast television over the past fifty years to the present time. However, this way has served the test of time and is not easily departed from. A much more complete discussion of television (for black and white as well as color) together with the timing, blanking, synchronizing (sync) signals, etc. required by the NTSC "standard" is given in a book entitled Basic Television and Video Systems, by Bernard Grob, published by McGraw-Hill, Inc., Fifth Edition, 1984.
CCD image sensors are well known in the art. Briefly described, a CCD image sensor has horizontal lines and vertical columns of light-sensing cells closely spaced within a given area onto which an image of a scene is optically focused. By way of example, in a CCD image sensor intended for high resolution still picture imaging, there are a thousand or more such cells in each horizontal line and in each vertical column for a total of a million or more cells within an area which may be only a few square centimeters. Each cell represents a very small area, termed a pixel, of the total image; the more pixels present in the CCD image sensor, the finer the resolution (or apparent lack of "grain") in the image reproduced by the CCD image sensor. By way of comparison, there are only about one-quarter-million pixels in a standard television image and, particularly when viewed as a still picture, the "grain" is apparent.
A CCD image sensor may have at the beginning of each horizontal line of cells a small number of cells (termed "Z ref" cells) used for determining a zero signal level. There are also a small number of cells (termed "D ref" cells) for determining a "dark" signal reference level, followed by a large number of "active" cells in the line for producing pixel image signals, and finally at the end of the line there are a few additional "Z ref" cells. One such CCD image sensor commercially available from the Eastman Kodak Co. (Part No. KAI-1000) has a total of 1032 cells in each horizontal line, with 2 "Z ref" cells at the beginning of the line, followed by 10 "D ref" cells, followed by 1014 "active" cells, followed by 6 "Z ref" cells at the end of the line, a total of 1032 cells. There are 1024 horizontal lines of these cells arranged in vertical columns. This is a much larger number of lines per picture frame than the number provided for by the NTSC standard (i.e., 525 horizontal lines per vertical picture frame). This much larger number of horizontal lines provided by such a high resolution CCD image sensor and the need for a sequential mode of operation causes problems in controlling the sensor in accordance with a television standard calling for considerably fewer lines, which lines must also be interlaced in "odd" and "even" fields in each picture frame.
When a high resolution CCD image sensor, such as described above, is used for still picture imaging, the preferred practice is to output the pixel image signals a line at a time in a straight sequence from the first line to the last line of a frame. This sequential outputting, rather than the outputting first of all of the lines of an "odd" field followed by all of the lines of an "even" field, as in standard TV, facilitates the subsequent utilization of the sequential video signals with existing apparatus (e.g., "photo CD" units, color printers, etc.) optimized to provide high resolution reproduction of individual pictures.
It is highly desirable, when producing pictures with an electronic camera, to be able visually to preview the actual pictures being imaged to see that the pictures are properly composed, have balanced light, etc. This is most conveniently done in real time by a viewfinder having a miniature video display. Miniature video display viewfinders operating in accordance with a television standard (i.e., interlaced lines of "odd" and "even" fields of a picture frame) are widely used in consumer camcorders and are very inexpensive. It is desirable therefore, from the standpoint of cost, to be able to use a standard camcorder viewfinder in conjunction with a high resolution CCD image sensor while maintaining the high-resolution capability of the sensor in generating picture images.
Operation of a CCD image sensor is well known in the art and is described briefly hereinbelow to provide a better appreciation of the present invention. The active cells of the CCD image sensor have their stored image signals (each of which corresponds to the light intensity of a small portion of an image) "read" out pixel-by-pixel, line-by-line to provide an electronic video image of a scene. Associated with each column of cells in a CCD image sensor is a separate vertical shift register.
In a sequential mode of operation, which as explained above is the preferred way of operating a high resolution CCD image sensor, at a selected instant of time the pixel image signals then stored in the horizontal lines of cells are simultaneously shifted into respective memory positions of the vertical shift registers. The simultaneous shifting of the multitude of individual pixel signals stored in the CCD cells of the horizontal lines into the respective vertical registers takes place within a short time termed "vertical blank" interval. The pixel signals thus stored in the vertical registers represent all of the horizontal lines of a single vertical picture frame. The pixel signals stored in all of the vertical registers are next shifted down in parallel at precise intervals within the vertical registers horizontal line by horizontal line and into respective memory positions of a line pixel register (horizontal shift register). There is a memory position in the line pixel register for each one of the vertical registers.
After a single horizontal line of pixels from the vertical registers has been shifted into the line pixel register, the image pixels of that horizontal line are clocked out of the line pixel register by a precisely determined cycle of timing pulses (hereinafter termed "pixel clock"). The pixel image signals thus outputted from the line pixel register are applied to other circuitry, such as an analog signal processor (ASP) as is well known in the art. The number of timing pulses in a cycle of the pixel clock corresponds to the number of cells in each horizontal line of cells in the CCD image sensor. This will be explained in greater detail below.
After all of the horizontal lines of pixel image signals of a given picture frame have been shifted into and clocked out of the line pixel register, the pixel image signals for the next frame of a picture stored on the cells of the horizontal lines of the CCD image sensor are simultaneously shifted into the vertical registers and the above-described sequence is repeated line by line for the entire frame. This sequential outputting of image signals of each frame is repeated in succession by precisely synchronized vertical and horizontal control signals applied to the CCD image sensor.
As is well known, a television frequency sub-carrier signal (hereinafter termed "fsc") provides for the decoding and display in proper sequence of the color-components (e.g., red, green and blue) of standard television image signals. This is also explained in detail in the above-identified book by Bernard Grob. To synchronize the pixel image signals in each horizontal line of cells of a CCD image sensor with a television standard, the number of cells in a horizontal line is made a convenient multiple of the television frequency sub-carrier ("fsc"). This will be explained in greater detail hereinafter. For the NTSC "standard", the "fsc" is 3.5795 MHz.
The synchronizing (sync) and control signals for a standard television system (e.g., NTSC) are well suited to the needs of video monitors such as used in camcorder viewfinder displays. Generic standard timing generators specifically designed for producing these "standard" sync and control signals are commercially available off-the-shelf at low cost from a number of companies. However, the standard sync and control signals produced by these commercially available timing generators are not directly usable as the vertical and horizontal control signals needed for a high resolution CCD image sensor, such as described above.
It is desirable from the standpoint of cost and convenience to be able to use such a generic standard timing generator and a miniature video display viewfinder, both of which are readily available commercially, in a high resolution imaging system where sequential readout of the lines of video signals of a CCD image sensor is required.
It is also desirable to have a simple, inexpensive and versatile imaging system which incorporates a television standard timing generator and a standard viewfinder display along with a high resolution CCD image sensor. The system should provide vertical and horizontal control signals for purely sequential, high resolution readout of the lines of video signals of the CCD image sensor and, alternatively, control signals as needed for viewing in real time of video images from the CCD image sensor in the viewfinder display.